Sound translating device



April 17, 1934. THURAs 1,954,966

SOUND TRANSLATING DEVICE Filed Jan. 31, 1931 I 2 Sheets-Sheet 2 INVENTORA. L. THURAS Afro/mgr :Patented Apr. 17, I934 1.954.966 SOUND TANSLATING DEVICE Albert L; Thuras, Bell Telephone York, N. Y.,

New York, N. Y., assignor to Laboratories, Incorporated, New acorporation of New York Application January a1, 1931, Serial No. 512,578

. This invention 20 Claims.

' vices and more particularly to such devices of the electrodynamic typefor tra ing sound waves. quality and transmission nsmitting or receiv-An object of the invention is to improve the characteristics oftelephone transmitters or receivers over substantially the whole audiblerange.

- In accordance with this invention, an electro- 10 dynamic devicecomprises 'a magnet struct re having coaxial pole pieces separated byanannular gap and supporting a piston type diaphragm carrying a movablecoil which is centered in the annular gap. The coil is adapted tovibrate with '15 the diaphragm to change the constant magnetic flux inthe annular gap in which the coil is suspended. The diaphragm iscontrolled in its movement by providing phrazm and utilizing a chamberunder the diathe mass. elasticity and viscosity of the air toefiiciently damp the mechanical motion of the at the side of the phragmand coil with a dam diaphragm. This chamber is provided by substantiallyclosing the annular gap pole pieces opposite the diapins plate.

Arr auxiliary damping element is formed in the pole piece to cooperatewith the ovide a substantially over the important audible frequencyrange for a device employed in transmitting or receivingsound waves.

In one embodiment of the invention, the main damping element is attachedto the annular pole piece and forms therewith a restricted radialpassageway entering the annular gapbetween' the I pole pieces. A sealingring carried by the central pole piece closes the gap between thecentral pole piece and' the damping .plate to restrict the pas- I I sageof air from the annular gap to the radial passageway between the dampingplate and the anconsists of a recessed central 1 nular pole piece. Theauxiliarydamping element pole piece fitted with 'a closure member havingsmall apertures to constitute an acoustic compensator sired values ofstiffness, mass,

having the de- 5 which imposes an added stiffness on the. diaphragm overthe low frequency range, then acts substantially as a resistance overthe frequency range where the diaphragm and chamber under the diaphragmproduces distortion and at hi h 1 5 frequencies acts'as a mass reactanceso that the movementof air in the chamber is toward the restrictedradial openingin the damping plate.

A further feature of the the constants of th invention relates to eauxiliary damping e'Pent compensator by providing a unit and resistanceI (Cl. 179-11555) relates to sound translating dewhich may be removableand adjustable to insure the correct values of mass, stiffness andresistance to alter the operating characteristics of the device in whichit functions.

These and other features of the invention will. be more fully understoodfrom the following de-' tailed description in connection with theaccompanying drawings. 5

Fig. 1 is a front elevation of an electrodynamic, device made inaccordance with this invention with portions cut away to show thesuperposed elements and a portion of the base shown in cross-section toshow the connection'of the leads from the movable coil to the stationaryterminals.

Fig. 2-isa cross-sectional view of Fig. 1 on the 7 line 2, 2 showing theassembly of the various elements entering into the structure of thedevice and the relationship of the main and auxiliary damping elementsin accordance with this inven- I tion. 7 Fig. 3 is an enlarged detailedview shown in cross-section of a partial assembly of. Fig. 2 to show thecooperating damping chambers under the diaphragm. 4

Fig. 3A is a plan view of the damping plate shown in Figs. 2, 3 and 8.

Fig. 4 is an enlarged plan view of the central pole piece which formsthe auxiliary damping element.

Fig. a is across-sectional view of Fig. 4 on the line 5, 5.

Fig. 6 shows in cross-section, a modification of the invention in whichthe central pole piece is provided with an annular restricted passagewayentering the auxiliary damping chamber.

Fig. '7 illustrates in cross-section, another modiilcation of theinvention inwhich the restricted passageway is provided by a thinspacing member Fig. 8 illustrates in cross-section a modified structureof the electrodynamic device shown in Fig. 2 in which the auxiliarydamping element is removable and adjustable; and

Fig. 9 shows the form of spacer disclosed in the auxiliary dampingelement of Figs..7 and 8. 1

'Referring now to Figs. .1 and 2, an electro- 1 dynamic transmitter isshown comprisinga mushroom-shaped casing having a magnet shell 10preferably of cobalt steel and an integral central pole 11 to which isattached a circular extension Q 12. An annular flanged ring 13. ofnon-magnetic material, such as phosphor bronze, is fitted over thecircular boundary of the extension 12 and seats against the lowertapered surface of the extension 12. A dome-shaped central pole piece 14is secured to the extension 12 and sesurely fastens the annular ring 13to the extension. An annular plate or polepiece 15 is coaxiallypositioned on the magnet shell with respect to the central pole piece 14by pins 16 held in soft metal inserts 17, the annular pole piece andcentral pole piece 14 forming therebetween an annular gap.

An annular damping plate or ring 18 of non-- magnetic material, suchasbrass or aluminum, 9. plan view of which is shown in Fig 3-A, isattached to one side of the annular pole piece 15 within .the casing-atthree spaced positions where the attachingscrew 19 extends through anoutwardiy extending portion 20 of the damping plate. This plate isprovided with an undercut surface having an annular ridge which forms arestricted annular passageway between the surface of theannular polepiece 15 and the damp ing plate 18. A sealing washer 21 of pliablematerial, such as paper, is located between the surfaces of the dampingplate 18 and the flanged non-magnetic ring 13, to seal the gap betweenthe irmer edges of the damping ring 18 and the flanged ring 13 so thatany movement of air in the chamber between the pole pieces must pass-.the central pole piece"14 and an intermediate corrugated or flexibleportion 24. A' movable,

coil 25 is attached to the diaphragm assembly intermediate the flexibleportion 24 and the dished rigid portion 23. and is adapted to movefreely in the annular gap between the edges of the annular pole piece 15and-thecentral pole piece 14 to change or modify the constant magneticflux in the annular gap between the pole pieces. The annular coil 25 mayconsist of a plurality of turns of ribbon conductor ound edgewise withadjacent turns insulated from each other by a coating of insulatingvarnish. The diaphragm assembly is spaced from the pole pieces by anumber of paper washers 26, to provide an air space between thediaphragm and the pole pieces which serves as a cushion for the movementof the diaphragm. The diaphragm assembly is secured to the annular polepiece 15 by screws threaded into a ring member 27 pro-' vided with asurface converging toward the diaphragm. A wire screen cloth 28 islocated over the opening of the ring27 and is held in place by a covermember having a central portion 29 provided with symmetrical cut-outportions 30 to allow the passage of sound to the diaphragm surface, anannular flanged portion 31 extending to the periphery of. the magnetshell and an overhanging flange portion or rim portion 32 which isforced over a shoulder of the shell as shown at 33. 3 A base mounting 34is attached to the periphery of the shell as shown inFig. 1 and providedwith a st tionary terminal block 35 to which the, leads 6, only onebeing shown, from the-movable coil in the "gap are attached. The

bottom of the base is closed by a plate member 3'1. i

In electrodynamic devices of the movable coil: type having a diaphragmof such massand stiffness as to operate as a piston when vibrating, themechanical movement of the diaphragm and coil causes distortion over acertain portion of the speech frequency'range, due to the naturalfrequency of the diaphragm being within this portion of the frequencyrange. This is observed by the shape of the characteristic curve whichhas a decided slope at the higher frequencies. By utilizing the mass,stifiness and reber on one side of the diaphragm, the air beingpermitted to leave the chamber through arestricted passageway, it ispossible to control the sistan'ce of a volume of air confined in a cham-I mechanical motion of the diaphragm and thereby reduce the distortionof the moving system ov substantially the whole frequency range. This isaccomplished by the main damping plate or ring 18 which introduces thenecessary amount of mass, stiffness and resistance by the movement ofthe air in the chamber under the dia-' phragm to overcome the distortionof the diaphragm. Y

The acoustic chamber formed under the diaphragm by the damping plate 18adds stiffness to the diaphragm and therefore raises the frequency ofthe diaphragm to a higher value. Due to the restricted passageway.leading from the large air space within the shell magnet to the chamberunder the diaphragm air is drawn through the passageway when thediaphragm is moved away from the pole pieces and likewise is forced outthrough the passageway when the diaphragm moves toward the "magneticstructure. By properly proportioning-the area of the passageway, it ispossible to introduce an acoustic impedance to the air entering andleaving the chamber under the diaphragm whereby the mechanical motion ofthe diaphragm is controlled over substantially the whole operatingrange, which is approximately from 60 cycles to 10.000 cycles. aForcertain acoustic devices such as electromagnetic and electrostaticreceivers and carbon microphones, this impedance should vary inverselyas the frequency. In a translating de-:

vice, such as a transmitter, as herein disclosed, this acousticimpedance should be constant with frequency over substantially the wholefrequency range. FF

- However, it has been found that the mass of the diaphragm and:stiffness of the air in the chamber under the diaphragm, cause somedistortion of the characteristic in the portion of the frequency rangebetween 2,000 and 6,000 cycles where the curve slopes slightly, thenrises above the desired flat characteristic. This condition is caused bythe acoustic impedance imposed on the diaphragm being too high for themass of the diaphragm.

In accordance with this invention, the mass. reactance of the diaphragmis neutralized over the range of frequencies in which distortion occursby introducing an auxiliary damping means or acoustic compensator underthe diaphragm which cooperates with the main damping means to smooth outthe characteristic curve of the device within a portion of the frequencyrange. This is accomplished in one form, as shown in Fig. 2, in whichthe central pole piece 14 is centrally recessed to form a hollow chamber38 between the pole piece 14 and the central pole extension 12.Communication with this chamber is secured by a plurality ofsmallvapertures 39 extending from the curved surface of the pole piece14 to the chamber 38. These apertures may be arranged in a circle aroundthe center of the pole piece, as shown in Fig. 4, and proportioned tointroduce the necessary amount of resistance to the airforced throughthe apertures by the motion of the diaphragm. The apertures 39 also.

form a communicating passageway between the chamber 38 in the centralpole piece and the chamber under the diaphragm which leads out throughthe restricted passageway in the main damping element 18.

In operation, the acoustic compensating means merely adds stiffness tothe diaphragm over the low frequency range up to, 2,000 cycles, th'enoperates substantially as a resistance in a region about 5,000 cycles inwhich the diaphragm mass distorts the characteristic and at higherfrequencies acts as a mass reactance. Since the main damping elementoffers a constant resistance over substantially the whole frequencyrange, it will be seen that the increasedresistance of the compensatingmeans neutralizes the mass reactance of the diaphragm to smooth out theslope and-rise in the characteristic curve of the device so that theoperating efficiency of the diaphragm is substantially a flatcharacteristic over thewhole frequency range.

Fig. 6 shows a modified structure of the acoustic compensating means inpiece 40 is provided with a large central aperture having a bridgemember 41 extending across the aperture within the hollow chamber. Theaperture is covered by a dome-shaped closure member 42 having acentralstem 43 which extends through the'bridge member 41 and is attachedtheretoby a nut 44. The closure member 42 is provided with a taperededge 45 to correspond with the machined surface of the aperture andsuitable spacers may be positioned between the surfaces of the closuremember 42 and the pole piece 40, to insure an accurate centering of theclosure member 42 whereupon it is rigidly fastened in place iby the nut44. The spacers may be'removed after securing the closure member inposition and the annular restricted opening or passageway formed betweenthe surfaces of the closure member 42 and the pole piece 40 insures thenecessary amount of resistance coupled to the stiffness and mass of theair in chamber within the pole piece. In Fig. 7 another modification ofthe acoustic the acoustic compensating means is shown in which a closureI member 46 is made of larger diameter than the aperture in the centralpole piece 40 and a spacing disc 47 having projections 48 is interposedbetween the adjacent surfaces of the closure member 46 and the polepiece 40. In this construction the projections 48 of the spacing member47 insure an accurate space relation 'between the pole piece 40 and theclosure member 46 and likewise insure equal and constant spacing betweenthe pole piece 40 and the closure member 46 whereby restricted slots areformed between the adjacent projections 48 on the spacing member 47..

Since individual diaphragm assemblies have diflerent characteristics, itis extremely desirable that provision be made to associate individualwhich a central pole 3 vice. In accordance with structure is disclosedin Fig. 8 ,in which the de-' this invention this vice comprises a magnetshell 50 having a central pole piece 51 attached thereto and centered bya pin 52. An annular pole piece 53 is coaxially centered around the onthe magnet structureby pins 54, the central pole piece 51 and theannular pole piece 53 forming an annular gap therebetween. An annulardamping plate or ring 55, similar to the ring 18 disclosed in Figs. 1and 2, is attached to the lower surface of the forms a plurality ofrestricted slots 56 between the damping plate and the annular pole piece53 due to .a spacing washer a being interposed between the ex! ensionsof ring 55 and the, annular pole piece 53. The damping plate liesagainst a soft rubber ring 57 held on the central pole piece 51 whichseals the gap between the inner surface of the damping plate and thesurface of the pole piece 51 so that the air from the chamber under thediaphragm must travel out through the restricted slots 56 between thedamping plate and the annular pole piece. A diaphragm assembly 58 isheld between spacing washers 59 and 60 and attached to the annular polepiece 53 by screws 61, extending through an annular retaining ring 62placed on top of the diaphragm assembly. An annular movable coil 63 isatiached to the diaphragm and moves freely in the annular gap betweenthe surfaces of the annular .pole piece and central pole piece 51.

The central pole piece 51 is as shown at 64.

The threaded cavity of the central pole piece 51 provides for the readyinsertion of the damping element or acoustic compensator member whichconsists of a cup-shaped member 65 provided wi h a flange 66, thecup-shaped member being threaded to enter the threaded portion of thecavity and the flange 66 lying against the top of the pole piece 51. Thecup-shaped member 65 is provided with a plurality of holes 67 in thebot'om thereof which are closed by a removable plate 68. Across the openend of the cup-shaped member 65 is positioned a dome-shaped closuremember 69 which is spaced from the edge of the cup-shaped pole piece 51and supportedannular pole piece 53 and member 65 by aspacing washer 47,the whole cavity in the cup-shaped member and the air resistance valueof the restricted slots between the closure member 68 and the wall ofthe cup-' shaped member 65. When the proper values are ascertained theclosure 68 is secured to the bottom of the damping element and thedamping element may be inserted in the central pole piece 51. The magnetassembly is enclosed in a metallic casing 71 andis secured therein by anannular flanged ring 72 which is threaded to the casing 71. The flangedring also secures a perforated screen 73 across the diaphragm surface toprevent the entrance of dust and to protect the diaphragm'from i jury.

While the invention has been disclosed in various modifications asapplied to an electrodynamic device, it is, of course, understood thatthe auxiliary damping or acoustic compensating means may beassociatedwith a main damping elementin other devices, and, therefore, theinvention is within the scope oi the apmeans associated with said magnetstructure for an air gap damping the mechanical motion of said diaphragmover substanially the whole operating range, and an acousticcompensating means adjacent said. diaphragm for influencing saiddiaphragm within a portion of said range.

2 A sound translating device comprising a magnet structure anddiaphragm, damping means associated with said magnet structure fordamping the mechanical motion of said diaphragm over substaniially thewhole operating range, and an acoustic compensating means adjacent saiddiaphragm, said means having the resistance, stiffness and massreactance of such values as to change the acoustic impedance imposed onthe diaphragm at any desired portion of the frequency range above 1,000cycles.

.3. A sound translating devicecomprising a, magnet having a central polepiece, a pole piece surroundingsaid central pole piece and formingtherebetween, a diaphragm in operative relation with respect .to saidpole pieces, a

coil attached to said diaphragm and located in said gap,.main dampingmeans associated with said magnet structure for damping the mechanicalmoiion of said diaphragm over substantially the whole operating range,and compensat ing damping means adjacent said diaphragm for controllingthe motion of said diaphragm within a portion of said range.

4. A sound translating device comprising a magnet having a central polepiece, an outer pole piece surrounding said central pole piece andforming an air gap therebetween, a diaphragm attached to said outer polepiece, a coil car ied by said diaphragm and located centrally in the gapbetween said pole pieces, a damping plate attached to said outer polepiece under said diaphragm to control the mechanical motion of saiddiaphragm over substantially the whole audible frequency range, andcompensating damping means formed in said central piece to control saiddiaphragm within a portion of said audible range.

5. A sound translating device comprising a magnet having a central polepiece. an annular pole piece on said magnet forming an annular gap withsaid central pole piece, a diaphragm supported on oneside of saidannular pole piece, an annular coil attached ta said diaphragm andmovable in said gap, and plurality ot damping means located on one sideof said diaphragm adjacent said coil, one of said means being centraland cylindrical and the other being annular and surrounding said centralmeans.

6. A sound translating device comprising a magnet having a central polepiece, an outer pole piece on said magnet forming an air gap with saidcentral pole piece, a diaphragm supported on one side of said outer polepiece, a coil attached to said diaphragm and movable in said gap, and aplurality of coaxial damping means associatedwith said magnet polepieces and located on one side of said diaphragm. v

7. A sound translating device comprising a magnet having a central polepiece, an outer pole piece on said magnet forming an air gap with saidcentral pole piece, a diaphragm supported on said outer pole piece, acoil attached to said diaphragm and movable in said gap, and

ing a dish-shaped portion supported on said a plurality of damping meanson one side of said diaphragm having restricted peripheral openingstransverse to said gap, said openings being in parallel planes.

8'. A sound translating device comprising a magnet having a, centralpole piece, an outer pole piece on said magnet forming an air gap withsaid central polepi'ece, a. diaphragm supported on said outer polepiece,,a coil attached to said diaphragm and movable in said gap, and aplurality of damping means on one side of said diaphragm, one of saidmeans being supported within said central pole piece and the other beingsupported by. said outer pole piece.

and the other surrounding said central pole piece.

-10. A sound translating device comprising a magnet having a centralpole piece, an outer pole piece on said magnet forming an air gap withsaid central pole piece, a diaphragm havouter-pole piece, a coilattached to said diaphragm and movable in said gap, and a plurality ofdamping means on one side of said dia-'- phragm, one of said meansforming 'a restricted path adjacent said outer pole piece and another ofsaid means forming a chamber and a'restricted path adjacent thedish-shaped portion of the diaphragm. I a

11. An electrodynamic device comprisingi a diaphragm and a-magneticstructure, one side 01 said diaphragm and walls of said magneticstructure forming a chamber, and means forming a plurality ofsubstantially annular openings for the ingress and egress of air intosaid chamber.

12. An electrodynamic device comprising a' sufllciently rigid to vibrateas a whole and a flexible portion, a coil attached to said diaphragm andsuspended in an air gap between the poles of said magnet. structure.damping means within said magnet structure adjacent said diaphragm atits flexible portion and coil, and auxiliary dampingmeans close to saiddiaphragm at its rigid portion. said coil being positioned substantiallyin a. line between said auxiliary damping means and said first dampingmeans.

13. An electrodynamic device comprising a magnet structure having arecessed central pole piece. an outer pole piece engaging said magnetstructure and forming an air gap with said central pole piece, adiaphragm, a coil, a damping, plate surrounding saidcentral pole pieceand substantially closing said gap, and a removable damping means withinsaid recessed central pole piece.

14. An electrodynamic device comprising a magnet structure having arecessed central pole: I

piece. an outer pole piece engaging said magnet structure and forming anair gap with said central pole piece, a diaphragm associated with saidtral pole piece and substantially enclosing said 150.

gap, and an adjustable damping member within saidrecessed central polepiece.

15. An electrodynamic device comprising a magnet structure having arecessed central pole piece, an annular pole piece engaging said magnetstructure ,and forming an annular gap with said central pole piece, adiaphragm mounted on one side of said annular pole piece, a coilattached to said diaphragm and located in said annular gap, a stationarydamping plate attached to the opposite side of said annular pole pieceand forming a chamber under said diaphragm, said damping plate havingrestricted radial openings communicating with said chamber, and aclosure member fitted over the open end of said recessed central polepiece and having peripheral slots communicating with said recessed polepiece and the chamber under said diaphragm.

16. In a constant impedance device, a magnet structure having polepieces forming an air gap therebetween, a diaphragm associated with saidpole pieces having a portion sufliciently rigid to vibrate as a whole, acoil attached to said diaphragm and suspended in said gap, a dampingmember attached to one of said pole pieces to form an air chamber in therear of said diaphragm, said damping member offering a constantresistance over substantially the whole audible frequency range, and anauxilary acoustic compensator memberclose to the central portion of saiddiaphragm, said member presenting sub-, vstantiallya resistance over aportion of said frequency range to neutralize the mass reactance of saiddiaphragm.

17. In a constant impedance device, a magnet structure having polepieces forming an air gap therebetween, a diaphragm supported 'on oneside of said pole pieces, said diaphragm having a portion suflicientlyrigid to vibrate as a whole, a coil attached to said diaphragm andsuspended in said gap, a damping member supported on one of said ,polepieces opposite said diaphragm and forming a chamber under saiddiaphragm having a constant resistance over a frequency range from 60 to10,0 00' cycles, and an acoustic compensator close to the centralportion of said diaphragm, said compensator acting substantially as aresistance within a range in the neighborhood of 6,000 cycles.

18. An electrodynamic device comprising a shell magnet having a centralpole, a hollow pole piece attached to said central pole, an annular polepiece surrounding said hollow pole piece, but spaced therefrom to forman annular gap between said pole pieces, a diaphragm attached to onesurface of said annular pole piece and-having a dome-shaped portionlocated over said hollow pole piece, an annular coil attached to saiddiaphragm and movable in the gap between said pole pieces, said polepieces and diaphragm forming walls oi an air chamber, an annular dampingplate attached to said annular pole piece having radial peripheralopenings communicating said shell magnet with said chamber, sealingmeans closing the opening between said damping plate and said centralpole, and an auxiliary damping plate closing said hollow pole piecehaving openings forming restricted passageways between said chamber andhollow pole piece.

19. An electrodynamic device comprising a magnet shell having a centralpole, a flanged ring on said central pole, an annular pole piece on saidmagnet, a central pole piece attached to said central pole and formingan annular gap with said annular pole piece while maintaining saidflanged ring on'said central pole, a diaphragm carried by said annularpole piece, a coil on said diaphragm located in said annular gap, adamping plate attached to said annular pole piece substantially closingsaid annular gap, said damping plate forming restricted radial slotsbetween said plate and annular pole piece, andsealing means interposedbetween said damping plate' and flanged ring.

20. An acoustic device comprising a magnet having a pair of polesforming an annular air gap,

a diaphragm, a coil secured to said diaphragm for movement in said gap,and means including one 01' said poles and said diaphragm forming an airchamber on one side of said diaphragm, said one pole having an aircavity therein and having bores connecting said air chamber and said aircavity.

ALBERT L. THURAS.

